The present invention relates to an image forming apparatus such as a copy machine with a variable copy magnification.
In a general copy machine, a moving exposure lamp optically scans a document on a document table. A rotating photosensitive drum is subjected to a slit exposure by the reflecting light from the document. Then, the picture on the document is imaged onto the photosensitive drum. The image formed on the drum is transferred to a copy paper. To change a copy magnification, it is necessary to change the size of the image on the drum. Generally, the density of a copy image on the copy paper is proportional to the density of the image on the drum. The density of the copy image is expressed by a ratio of the amount of exposure light emitted from the exposure lamp to the size of the image on the drum. This indicates that, with a fixed amount of exposure light, the copy image is darker with the increase of the copy magnification, and vice versa. Thus, the density of the image copied changes with the setting or selecting of the copy magnification. For fixing the density of the copy image independent of the copy magnification, it is necessary to control the amount of exposure light according to the copy magnification, allowing for the above relationship between the copy density and the copy magnification.
The amount of the exposure light emitted from an exposure lamp is generally controlled by a lamp regulator. The lamp regulator includes a control signal generator for generating a control signal upon the detection of reflected light from a document, and a detector for detecting a voltage applied to the exposure lamp. A difference between the control signal and the applied voltage is used for controlling the voltage applied to the exposure lamp. Accordingly, the density of each document defines the reference signal. In this way, the amount of exposure light from the exposure lamp is automatically controlled.
The control signal is obtained, for example, by amplifying the output signal from a photo sensor element by a differential amplifier. To make the control signal also dependent on the copy magnification, in such a case, it is sufficient to change the amplification factor of the differential amplifier. The amplification factor of the differential amplifier is proportional to a feedback resistance. This implies that, to change the amplification factor, a number of feedback paths with different resistances are provided, and one of the feedback paths is selected according to the copy magnification. This approach effectively works for a lesser number of selected copy magnifications. However, it fails to cope well in the case when the copy magnification must be continuously changed, because of its inevitable size increase. In the conventional copy machine, only a few control signals are available for controlling the exposure light even if the copy magnification may be continuously changed. Hence, these signals cannot correctly control the exposure light for every copy magnification selected.